Adenocarcinoma of the prostate gland is the second most common cancer in males over the age of 55. Lee et al "Transrectal Ultrasound in the Diagnosis of Prostate Cancer; Location, Echogenicity, Histopathology, and Staging", The Prostate 7:117-129 (1985). It has been shown that transrectal ultrasound can be used to detect cancer of the prostate gland as well as to demonstrate the extent of tumor involvement so as to enable accurate staging of the detected cancers. Id., Lee et al "Needle Aspiration and Core Biopsy of Prostate Cancer: Comparative Evaluation of Biplane or Transrectal US Guidance", Radiology 1987; 163:515520. Prostate endosonography has also been shown to be a potential screening test with the potential for improving the quality of life in patients affected by prostate carcinoma. A. Fleischer, "Prostatic Endosonography-a potential screening test", Diagnostic Imaging, page 78, 1987.
The aforementioned findings have been the result of the new use of ultrasound imaging and the acquisition of images of the prostate gland. These images are obtained by inserting a condom covered probe into the rectum and then inflating the condom with water. An ultrasound transducer mounted on the probe is then immediately adjacent to the small prostate gland and can transmit to and receive ultrasound signals from the prostate due to the water coupling. Because of this close proximity, high frequency, high quality, and small field of view images of the prostate can be obtained.
Recent efforts in this new imaging field have demonstrated that the small primary cancerous lesions that develop in the prostate's peripheral zone are clearly visible in the ultrasound images as hypoechoic regions. This means that not only is prostate cancer now visible for a study in all stages of development, but also that screening of the male population is possible.
The most recently developed equipment for ultrasound imaging is the biplane prostate probe. An example of such a prostate probe is the RT3600 Biplane Transrectal probe manufactured by General Electric Company, Medical Systems Group, Milwaukee, Wisconsin. Two ultrasound transducers are mounted on this probe. One is oriented to obtain a transaxial view of the prostate gland and the other is oriented to obtain a sagittal view. These two orthogonal views of the prostate have proved important in clinical imaging of the prostate and for ultrasound guided needle biopsies of the prostate. As stated in the aforementioned article by Lee et al in Radiology, experimental results suggest that transrectal ultrasound guidance of thin-needle biopsies is useful in diagnosing early prostate cancer.
With two transducers mounted in the biplane probe, the operator needs only to switch electrically between the two in order to change the plane of view. Previous to the development of the biplane probe, the operator was obliged to remove one probe from the patient's rectum and insert another before the operator could obtain a new view of the prostate. The biplane probe is more efficient to use and reduces the necessary examination time.
One important use of the biplane probe is the positive identification of suspected primary cancerous lesions in the peripheral zone of the prostate. Another important use is ultrasonically guiding the needle biopsy of lesions in order to accurately obtain a small tissue or cytology sample for pathological analysis. In both applications, it is essential for the operator to view the tissue area of interest in one scan plane and position it properly so that when the orthogonal scan plane is chosen, the area of interest is immediately viewed. This facility will permit the positive identification of a focal lesion in two orthogonal planes and will also permit the positive localization of the tip of the biopsy lesion using two orthogonal scan planes. The selection of the scan plane must also be made quickly and conveniently so that the operator does not move the probe in the patient's rectum during the switching procedure.
Most newly developed biplane probes consist of two transducers. One transducer is a linear array or phased array for sagittal imaging and the other is a phased array (or mechanical sector scanner) for transaxial imaging. These two transducers must be physically separated when mounted on the biplane probe. Usually, the transaxial transducer is mounted close to the tip of the probe for prostate visualization reasons. Since the sagittal scan plane field of view of a linear array is rectangular and the transaxial plane field of view is positioned to the side of the sagittal linear array, it is not possible to view the transaxial scan plane position (on edge) in the sagittal field of view. Thus, in using the biplane probe, the clinical user cannot switch from sagittal to transaxial scan planes and maintain a view of the tissue area of interest. After the transaxial scan plane is selected, the probe must be repositioned in the patient's rectum in order to visualize the tissue area of interest again. This is not only very inconvenient but drastically reduces the clinical utility of this biplane probe design.